Add formatter coverage tests

[RAprogramm]

Summary

• add runtime assertions covering every derive formatter variant and case-sensitive formatter output
• extend the formatter trybuild suite with per-formatter pass coverage plus compile-fail fixtures for uppercase-only specifiers
• bump the crate to version 0.5.12 and document the release in the changelog and README examples

Testing

• cargo +nightly fmt --
• cargo clippy -- -D warnings
• cargo build --all-targets
• TRYBUILD=overwrite cargo test --test error_derive_from_trybuild
• cargo test --all
• cargo test
• cargo test --test error_derive
• cargo doc --no-deps
• cargo audit
• cargo deny check

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https://chatgpt.com/codex/tasks/task_e_68cd103df864832b82f5dafd60afa5c0